How Long Do I Keep Red Leaf Maple Trees In Pots?

My brother, who lives in Chincoteague, VA, gave me 15 small (4-6 inches) red leaf maple trees. It took a week for me to come home and by then, I was able to save 9 of them. I have them in individual pots and they are now 12-14 inches above the soil. When should they be put into the ground? I live in northern Florida.